                Inkjet printers form a printed image by printing a pattern of individual dots at particular locations of an array defined for the printing medium. The locations are conveniently visualized as being small dots in a rectilinear array. The locations are sometimes referred to as “dot locations”, “dot positions”, or “pixels.” Inkjet printers print dots by ejecting very small drops of ink onto the print medium to fill a pattern of dot locations with dots of ink. The printers typically include a movable carriage that supports the printheads for movement over the surface of the print medium. Each of the printheads has an array of ink ejection nozzles that are controlled to eject droplets of ink at appropriate times pursuant to commands of a microcomputer or other controller. The timing of the application of the ink droplets is intended to correspond to the pattern of the pixels of the image being printed.        
Color inkjet printers commonly employ a plurality of printheads, mounted in the print carriage to produce different colors. Each printhead contains ink of a different color, with the commonly used colors being cyan, magenta, yellow and black. The various colors are produced by depositing droplets of the required colors onto dot locations. Secondary or shaded colors are formed by depositing drops of primary different colors on adjacent or overlapping dot locations with the human eye interpreting the color mixing as the secondary or shaded colors.
Print quality is one of the most important considerations of competition in the color inkjet printer field. As the image output of a color ink-jet printer is formed of millions of individual ink droplets, the quality of the image is ultimately dependent upon the quality of each ink droplet and the arrangements of the ink droplets on the print medium.
With reference to FIG. 1, a printhead arrangement 5 employed in the past has typically used printheads comprising linear arrays of print elements such as nozzles, wherein printheads of different colors are arranged one next to the other in a single row in the direction of the swath axis or scan axis, arrows A-B. This may be referred to as the so-called in-line case illustrated by printheads 5Y, 5M, 5C and 5K. There are several negative consequences of such an arrangement where bi-directional printing is employed. One is that, when the printer carriage is going from left-to-right, the colors are laid down in one order, say YMC for example. When the carriage goes in the other direction, from right-to-left, the colors are laid down in the opposite order, CMY in this example. The problem with this is that the blue made by first printing cyan and then magenta is slightly different from the blue made in the reverse order. This is because the final dot will inevitably cover a bit of the first dot. The slight color difference between the left-to-right printing and the right-to-left printing is called color hue shift. The color hue shift causes bi-directional hue shift banding, an outstanding image defect for inkjet printing. Of course, the blue color shifting is only one example as other colors will also suffer from hue shift banding in a similar fashion.
In order to overcome this problem, a fully staggered printhead layout may be employed wherein each of the printheads are staggered in the direction of the print medium advancement so that no two printheads are in the same row when moved in the direction of the print swath axis. An example of the fully staggered printhead arrangement 7 is shown in FIG. 2. In this arrangement, printheads 7Y, 7M, 7C and 7K are positioned so that they are spaced in the media advance direction, arrow C, and when operated for printing, no two printheads print dots on top of each other when moved together in one of the carriage advancements in the direction of the print swath axis, arrows A-B, within a swath. The resulting color sequence of printing one color upon another is independent of carriage movement direction. Therefore, there is no bi-directional hue shift banding. However, the fully staggered layout causes the length of the print zone to be significantly increased. For example, in FIG. 1, the print zone length L, of the in-line printheads layout is the same as the printhead height H. In FIG. 2 for the fully staggered printhead layout, the print zone length L, is at least four times that of the printhead height H. A large print zone length is not preferred because of the difficulty of maintaining a constant spacing distance between the printheads and the medium being printed upon. Also, a small angular error in the media feed axis results in greater relative dot placement errors between printheads. Where the printer includes additional printheads having different shades of at least some of the primary colors providing a fully staggered layout undesirably causes substantial increase to the print zone of the printer.